Mac: Don't enable dSYMs by default for non-offical builds

Building with dSYMs enabled results in substantially longer link times and
provides minimal benefit for builds that are not intended for distribution.

They can still optionally be enabled by setting enable_dsyms=true via
GN_DEFINES. They remain enabled by default for official builds generated
with is_official_build=true and are required if packaging symbols via the
make_distrib.py script.

BUILD.gn

@@ -261,12 +261,6 @@ if (is_clang) {
   assert(!clang_use_chrome_plugins)
 }
 
-if (is_mac) {
-  # Always generate dSYM files. The make_distrib script will fail if
-  # enable_dsyms=true is not explicitly set when is_official_build=false.
-  assert(enable_dsyms)
-}
-
 
 #
 # Local variables.

tools/gn_args.py

@@ -277,11 +277,6 @@ def GetRequiredArgs():
     # can't be enforced by assert().
     result['enable_linux_installer'] = False
 
-  if platform == 'mac':
-    # Always generate dSYM files. The make_distrib script will fail if
-    # enable_dsyms=true is not explicitly set when is_official_build=false.
-    result['enable_dsyms'] = True
-
   return result